On the digitizing of historical seismograms

Abstract

The pens of early mechanical seismographs record a curved rather than the straight line when the pendulum is shifted from the equilibrium position. Consequently, the abscissa of the seismogram is generally not a linear function of time. This can be avoided by simple corrections. The first one changes the digitized coordinates into those of the zero line of the seismogram, and the second one transforms the zero line coordinates to the nonlinear time scale. This process will preserve both amplitudes of displacement and the time history of the seismogram.